Particulate Fuel Combustion Method and Furnace

ABSTRACT

Method of operating a combustion furnace comprising at least one pair of particulate fuel burners positioned opposite one another across a combustion zone, whereby each burner of a pair operates in turn to generate a flame directed at the other inactive burner of the pair so as to locally overheat the burner block of the inactive burner.

The present invention relates to the field of particulate fuelcombustion and the use of particulate fuel burners in industrialfurnaces.

The present invention relates in particular to the use of particulatesolid fuel burners, such as particulate coal burners.

The present invention also relates to the use of particulate liquid fuelburners, whereby combustion is generated by injecting oxidant andparticulate liquid fuel, i.e. droplets of liquid fuel, into a combustionzone. The present invention relates more particularly to the use of suchparticulate liquid fuel burners for heavy liquid fuels.

Coal is the most abundant fossil fuel currently available. Most of thepower generated in the world uses coal as the fuel.

One way of generating heat or power is the use of coal burners.

In a coal burner, a conveying gas is often required to transport thesolid fuel particles from a fuel storage or milling device (e.g. a coalpulveriser) to the burner for subsequent combustion with an oxidant. Theoxidant for the combustion can be the conveying gas, a gas suppliedseparately from the conveying gas or a combination of the conveying gasand a separately supplied gas.

The combustion process of particulate solid fuel comprises severalcombustion steps which are described hereafter with reference to thecombustion of particulate coal:

-   -   1) The coal particles, which typically leave the storage or        milling device substantially at ambient temperature, are heated        to at least the devolatilization temperature of the coal. The        devolatilization temperature is the minimum temperature at which        devolatilization of the coal commences (see hereafter). The        devolatilization temperature may vary depending on the type or        grade (class) of coal, its humidity, etc.    -   2) When the temperature of the coal particles reaches the        devolatilization temperature of the coal, devolatilization of        the coal particles commences. Devolatilization is a process in        which the volatile components (in short:volatiles) of the coal        particles leave the coal particles in gaseous form. Volatiles        include highly combustible components such as hydrocarbons and        hydrogen.    -   3) The volatiles combust with the oxidant, thereby generating        heat and increasing the temperature of the remaining solid        matter or char.    -   4) Finally, the char combusts with remaining oxidant, thereby        generating further heat.

This multi-step combustion process distinguishes particulate solid fuelcombustion from the gaseous fuel combustion process in which the gaseousfuel combusts directly with the oxidant.

The particulate liquid fuel combustion process, in which liquid fuel isinjected into the combustion zone in the form of small particles ordroplets, is also a multi-step process. In a first step, the injectedliquid fuel droplets are heated to the evaporation temperature of thefuel when the fuel reaches its evaporation temperature, the liquid fuelevaporates to form inflammable fuel vapours and in the third step theinflammable fuel vapours combust with the oxidant and produce heat. Forlight fuels, such as domestic fuel oils, or No 1, 2 and 3 fuel oils, theevaporation temperature is relatively low and evaporation of the fuelinto vapours takes place almost instantly following injection into thecombustion zone at normal operational temperatures of most industrialfurnaces. Consequently, the combustion of particulate light liquid fuelsresembles that of gaseous fuels as far as rate of combustion followinginjection is concerned.

In the combustion of particulate medium heavy liquid fuels such as No 4fuel oil and very heavy liquid fuels such as residual fuel oil, or No 5and 6 fuel oils, the evaporation temperature is higher and evaporationof the liquid fuel takes place more slowly and more gradually. In thismanner, the combustion of particulate heavy and especially very heavyliquid fuels resembles the multi-step process of particulate solid fuelcombustion.

As a consequence, particulate solid fuel burners, such as particulatecoal burners, and particulate heavy liquid fuel burners, are usually notsuited for a narrow combustion chambers in which only short flames canbe used for heat generation.

Indeed, when the length of the flame exceeds the width of the combustionchamber (the width being the free dimension of the combustion chamberalong the flame axis), the flame impinges on the combustion chamberstructure (such as a chamber wall) opposite the burner, thereby causingincomplete fuel combustion and fouling with partial-combustion productssuch as soot as well as thermal damage to the impinged chamberstructure.

Air is traditionally used as the conveying gas and as the oxidant forparticulate fuel burners, as the conveying gas for solid particulatefuels and as the pulverisation gas for particulate liquid fuelinjectors. Burners using air as the oxidant for combustion are known asair-fuel burners.

In the case of oxy-fuel burners, the oxidant is an oxygen-rich gas (>25%vol O₂) such as oxygen-enriched air or industrial oxygen having anoxygen content of at least 90% vol, preferably of at least 95% vol, andmore preferably of at least 98% vol.

The advantage of oxy-fuel burners over air-fuel burners are multiple:

-   -   improved fuel efficiency    -   improved heat transfer towards the charge to be heated,    -   reduced fumes generation,    -   higher CO₂ concentration in the fumes, which is advantageous for        CO₂ capture and sequestration    -   reduced pollutants emissions (e.g. NOx . . . ),    -   higher flame temperature providing improved the combustion of        hard-to-burn fuels,    -   etc.

In the case of oxy-fuel burners, the risk of thermal damage to theinstallation in case of narrow combustion chambers is particularlyimportant due to the higher flame temperature when compared to air-fuelburners.

Examples of narrow combustion chambers are side and/or cross-firedtunnel or passage furnaces, such as cement passage kilns, glass feedersor forehearths.

In view of the high availability of solid fuels such as coal, includinglow-grade coal, and of heavy fuels, often at advantageous prices, itwould be highly desirable to be able to use particulate fuel burners inindustrial narrow combustion chambers.

This is accomplished by the method of operating a furnace of the presentinvention.

The furnace used in said method comprises a combustion chamber havingwalls defining a combustion zone within the combustion chamber.

The furnace further comprises at least one pair of particulate fuelburners. In the present context, each pair of burners consists of afirst particulate fuel burner and a second particulate fuel burner, bothsaid burners being equipped to inject fuel and oxidant into thecombustion chamber and comprise a burner block having at least onepassage for transporting fuel and/or oxidant through the burner blocktowards the combustion chamber.

The burner blocks of the first and second particulate fuel burners ofeach pair are mounted in the walls of the combustion chamber so as to bepositioned opposite one another across the combustion zone.

Both burners of each pair are adapted to generate a flame in thecombustion zone by:

-   -   transporting fuel and/or oxidant through the at least one        passage of the burner block, and    -   injecting the oxidant in gaseous form and the fuel in        particulate form into the combustion zone for combustion in said        combustion zone.

The method according to the invention comprises alternating first andsecond steps.

In the first step, the second particulate fuel burner of each pair doesnot generate a flame in the combustion zone, whereas the firstparticulate fuel burner of each pair generates a flame in the combustionzone directed at the second burner block so as to cause localoverheating of the second burner block of said pair.

In the second step, the first particulate fuel burner of each pair doesnot generate a flame in the combustion zone and the second particulatefuel burner of each pair generates a flame in the combustion zonedirected at the first burner block so as to cause local overheating ofthe first burner block of said pair.

According to a preferred embodiment of the present invention, which evenfurther reduces the risk of thermal damage to the installation in caseof narrow combustion chambers:

-   -   in the first step, the second particulate fuel burner of each        pair, which does not generate a flame in the combustion zone,        injects a deflecting gas into the combustion chamber so as to        partially deflect the flame generated by the first particulate        fuel burner of said pair back into the combustion zone, and        similarly    -   in the second step, the first particulate fuel burner of each        pair, which does not generate a flame in the combustion zone,        injects a deflecting gas into the combustion chamber so as to        partially deflect the flame generated by the second particulate        fuel burner of said pair back towards the combustion zone.

A number of gases can be used as the deflecting gas. The deflecting gascan, for example, be an oxidant. The deflecting gas can advantageouslybe flue gas, in particular flue gas recycled from the combustionchamber. Depending on the nature of the process, it may be indicated topartially or totally remove water vapour from the flue gas, e.g. bycondensation, before the flue gas is thus recycled. For otherapplications, the use of steam as the deflecting may be beneficial.

The fuel injected by the first and second particulate fuel burners ofthe/each pair can be a particulate solid fuel or a particulate liquidfuel.

Examples of suitable particulate solid fuel are particulate coal, petcoke, combustible solid particulate waste. Different classes ofparticulate coal may be used depending on the process: lignite,bituminous coal or anthracite, from highly coking to non-coking coals,etc.

Particular examples of suitable liquid fuels are medium heavy liquidfuels, such as No 3 fuel oil, and heavy liquid fuels such as No 5 and No6 fuel oils, furnace fuel oils (FFO) and certain combustible liquidindustrial wastes. The present invention is particularly useful for thecombustion of waste fuel or fuel waste, if appropriate for the processconcerned, in particular with regard to any effects on the charge to beheated.

Suitable particulate solid fuel burners are notably known fromWO200603296, WO2007063386, and from co-pending European patentapplication EP 09174622.2. Suitable particulate liquid fuel burners areknown from EP 1750057 and WO03006879.

The first and second particulate fuel burners of one pair, of severalpairs or of each pair may be oxy-fuel burners. In particular, the firstand second particulate fuel burners of one pair, of several pairs or ofeach pair may be oxy-fuel burners operating with an oxidant, such as forexample oxygen-enriched air, containing at least 50% by volume ofoxygen, preferably at least 80% by volume, more preferable at least 90%by volume of oxygen, or industrial oxygen having an oxygen content of atleast 95% by volume, and preferably of at least 98% by volume.

In many instances, the combustion chamber has a first wall and a secondwall, the first wall being positioned opposite the second wall acrossthe combustion zone.

In that case, the burner block of the first particulate fuel burner of apair can for example be mounted in the first wall and the burner blockof the second particulate burner of the pair in the second wall.

In this manner, when the furnace comprises a multitude of pairs ofparticulate fuel burners, the first particulate fuel burner of each pairmay be mounted in the first wall and the burner block of the secondparticulate burner of each pair in the second wall. In particular, thefirst wall of the combustion chamber may comprises a row of burnerblocks of first particulate fuel burners and the second wall a row ofburner blocks of second particulate fuel burners. According to thisembodiment of the invention, in the first step, flames are generated bythe burners mounted in the first wall and, in the second step, flamesare generated by the burners mounted in the second wall of thecombustion chamber.

Alternatively, some (i.e. a first portion) of the multiple pairs canhave the burner block of the first particulate fuel burner mounted inthe first wall and the second particulate fuel burner mounted in thesecond wall, while the remaining portion of the multitude of pairs hasthe burner block of the second particulate fuel burner mounted in thefirst wall and the first particulate fuel burner mounted in the secondwall. In particular, the first wall may have a first row of burnerblocks mounted therein and the second wall a second row of burner blocksmounted therein, so that said first and second rows are alternating rowsof burner blocks of first particulate fuel burners and burner blocks ofsecond particulate fuel burners, i.e. a block of a first burner followedby a block of a second burner followed by a block of a first burner,etc. According to this embodiment, flames are generated by some burnersmounted in the first wall and by some burners mounted in the second wallduring both the first and the second step.

The first and second walls are advantageously lateral walls of thecombustion chamber, in particular when the furnace has a long and narrowcombustion chamber as is usually the case with glass feeders orforehearths and reheat furnaces.

According to a first simple embodiment of the method according to theinvention, the first and second steps have a predetermined duration. Theduration of the first and second step is/are selected so as to provide,on the one hand, sufficient local overheating of the burner block so asto be beneficial for the subsequent step, while, on the other hand,preventing thermal damage to the burner block or other constituentburner parts.

For increased safety of operation, the burner blocks of the first andsecond burner of at least one pair may be equipped with a temperaturedetector. Said temperature detector may include a temperature sensormounted inside the burner block or may detect the temperature of asurface of a burner block facing away from the combustion zone.

In that case, the duration of the first and second steps may bedetermined as follows:

-   -   the first step is terminated and the second step is commenced        when the temperature detector of the burner block of the second        burner detects a temperature exceeding a first predetermined        upper limit, and    -   the second step is terminated and the first step is commenced        when the temperature detector of the burner block of the first        burner detects a temperature exceeding a second predetermined        upper limit.        The predetermined upper temperature limits are selected so as to        permit sufficient local overheating of the burner block, while        preventing thermal damage to the burner block or other        constituent burner parts.

According to an alternative embodiment:

-   -   the first step is terminated and the second step commences when        the temperature detector of the burner block of the first burner        detects a temperature lower than a first predetermined lower        limit, and    -   the second step is terminated and the first step commences when        the temperature detector of the burner block of the second        burner detects a temperature lower than a second predetermined        lower limit.        The predetermined lower temperature limits are selected so as to        switch method steps when the advantageous effects of the local        overheating in the previous step have worn off.

Advantageously, the method according to the invention combines two ormore of said approaches and the advantages thereof.

According to one such embodiment:

-   -   the first step is terminated and the second step commences when        one of the following criteria is met:        -   the temperature detector of the burner block of the second            burner detects a temperature exceeding a first predetermined            upper limit, and        -   the temperature detector of the burner block of the first            burner detects a temperature lower than a first            predetermined lower limit, whereas    -   the second step is terminated and the first step commences when        one of the following criteria is met:        -   the temperature detector of the burner block of the first            burner detects a temperature exceeding a second            predetermined upper limit, and        -   the temperature detector of the burner block of the second            burner detects a temperature lower than a second            predetermined lower limit.

Alternatively:

-   -   the first step is terminated and the second step commences when        one of the following criteria is met:        -   the temperature detector of the burner block of the second            burner detects a temperature exceeding a first predetermined            upper limit,        -   the first step has reached a predetermined duration, whereas    -   the second step is terminated and the first step commences when        one of the following criteria is met:        -   the temperature detector of the burner block of the first            burner detects a temperature exceeding a second            predetermined upper limit, and        -   the second step has reached the predetermined duration.

According to a further advantageous mode of operation:

-   -   the first step is terminated and the second step commences when        one of the following criteria is met:        -   the temperature detector of the burner block of the first            burner detects a temperature lower than a first            predetermined lower limit, and        -   the first step has reached a predetermined duration, whereas    -   the second step is terminated and the first step commences when        one of the following criteria is met:        -   the temperature detector of the burner block of the second            burner detects a temperature lower than a second            predetermined lower limit, and        -   the second step has reached the predetermined duration.            According to a further optimized possibility:    -   the first step is terminated and the second step commences when        one of the following criteria is met:        -   the temperature detector of the burner block of the second            burner detects a temperature exceeding a first predetermined            upper limit,        -   the temperature detector of the burner block of the first            burner detects a temperature lower than a first            predetermined lower limit, and        -   the first step has reached a predetermined duration,    -   the second step is terminated and the first step commences when        one of the following criteria is met:        -   the temperature detector of the burner block of the first            burner detects a temperature exceeding a second            predetermined upper limit,        -   the temperature detector of the burner block of the second            burner detects a temperature lower than a second            predetermined lower limit, and        -   the second step has reached the predetermined duration.

In general, the predetermined duration of the first and second step willbe identical. Likewise, the first and second predetermined uppertemperature limits will normally be the same. Similarly, the first andsecond predetermined lower temperature limits will be the same. However,different predetermined durations, different upper temperature limitsand/or different lower temperature limits may be selected when thefurnace configuration or the process parameters justify same, forexample when different burners are used as first burners and as secondburners.

Suitable particulate solid fuel burners, are notably known fromWO200603296, WO2007063386, and from co-pending European patentapplication EP 09174622.2. Suitable particulate liquid fuel burners areknown from EP 1750057 and WO03006879.

The method of the present invention is particularly advantageous fornarrow furnaces and in particular for tunnel furnaces. The benefits ofthe invention are particularly apparent when the furnace is a glassmelting furnace, a glass feeder or forehearth, a tunnel calcinationsfurnace or a reheat furnace.

The mechanisms and advantages of the methods according to the presentinvention are described in more detail hereafter, reference being madeto FIGS. 1 to 4, whereby:

FIG. 1 is a partial cross section of a narrow furnace equipped withseveral pairs of burners according to a horizontal plane across thepassages through the respective burner blocks,

FIG. 2 is a close-up view of burner pair II of FIG. 1 during step 1

FIG. 3 is a close-up view of burner pair II of FIG. 1 during step 2

FIG. 4 is a close up view of burner pair II of FIG. 1 during analternative embodiment of step 1.

The present invention makes it possible to obtain relatively shortparticulate fuel flames by reducing the residence time required for thefuel particles to reach their devolatilization, respectively theirvaporisation temperature after their injection into the combustion zone,thus shortening the distance travelled by the fuel particles in thecombustion zone before combustion of the volatiles, respectively of thefuel vapours commences, which in term leads to a shortening of theflame.

According to the present invention, this is achieved as follows.

During the first step, the flames 10 generated by the first burner 100of each pair is directed at the burner block 201 of the “inactive”second burner 200 of said pair so as to cause local overheating of saidburner block 201.

In the present context, “local overheating” a burner block refers to theheating of the burner block 101, 201 to a temperature higher than thetemperature of the furnace wall 150, 250 surrounding the burner block.The maximum temperature to which the burner block can thus be locallyoverheated is determined by the thermal resistance properties of theburner block and of other burner constituent parts, such as metallicinjectors mounted in or connected to said burner block.

In the respect, a preferred material for the burner block is AZS.

Fused cost AZS blocks have an important thermal inertia and high thermalresistance.

In order to prevent thermal damage to metallic parts of the burnerblock, these metallic parts are advantageously restricted to the rearhalf of the burner block, i.e. the metallic parts do not extend beyondhalf the width of the burner block starting from the surface of theblock facing away from the combustion zone.

A suitable choice of metal for (part of) the metallic parts can alsohelp prevent thermal damage. Examples of such suitable metals are heatresistant alloys such as Inconel® 600 and Kantal®.

The burner block can typically be heated to a temperature (on the sideof the block facing the combustion zone) of at least 1400° C.,preferably at least 1500° C. The burner block can advantageously beheated to temperatures of up to 1700° C. (on the side of the blockfacing the combustion zone), in particular in the absence of metallicparts in the burner block on the side of the block facing the combustionzone.

In the present context, a burner is said to be “active” when it injectsfuel and oxidant into the combustion zone and thereby generates a flamein the combustion zone and “inactive” when it does not generate a flamein the combustion zone.

During the first step, the burner block 201 of the second burner 200acts as a heat accumulator.

As illustrated in FIG. 3, when the second step commences, fuel and/oroxidant is transported through the at least one passage 202 of thesecond burner 200 of the pair and injected into the combustion zone 1 togenerate a flame 20. In what follows, fuel and oxidant are collectivelyreferred to as “reactants”. Fuel and oxidant can be injected through asame passage of the burner block. Alternatively, fuel and oxidant can beinjected through different passages or one or more passages can be usedto inject fuel and oxidant, whereas another or several other passagesare used to inject additional oxidant. When the fuel is transportedthrough a passage of the block by means of a conveyor gas, oxidant isadvantageously used as the conveyor gas.

As the at least one passage 202 is situated in the burner block 201 ofthe second burner 200, the heat accumulated in said burner block 201during the first step is released to progressively raise the temperatureof the reactant or reactants flowing therethrough towards the combustionzone 1 (fuel and/or oxidant preheating). As a consequence, the reactantis injected into the combustion zone 1 at a higher temperature thanwould have been the case without local overheating of said block 201 inthe preceding step. As a consequence, the injected fuel particles reachtheir devolatilization/evaporation temperature more rapidly, i.e. aftera shorter travel distance in the combustion zone.

This in turn leads to the start of combustion of the volatiles/fuelvapours after a shorter residence time and shorter travel distance sothat the overall flame length is likewise shortened. The flamesgenerated by the method according to the invention can therefore safelybe used for heating narrower furnaces.

During said second step, the flame 20 generated by the “active” secondburner 200 of each pair is directed at the burner block 101 of the“inactive” first burner 100 of said pair, so as to cause localoverheating of said burner block 101.

In this manner, analogous flame-shortening effects are obtained for theflame 10 generated by the first burner 100 of each pair in thesubsequent first step in which the first burner 100 becomes “active”again (see FIG. 2).

In that the local overheating by the burner flame generated by the“active” burner is limited to the burner block of the “inactive” burnerof the pair, excessive temperature and therefore thermal damage to thefurnaces wall surrounding the “inactive” burner is provided.

In that the reactant preheating takes place in the one or more passagesof the burner block directly upstream of the combustion zone, prematuredevolatilization/evaporation and ignition of the fuel in the burnersupply system is prevented.

In some cases, the flame shortening achieved in the above manner may notsuffice to shorter the flame to less than the width of the furnace.

According to a specific embodiment of the present invention illustratedin FIG. 4, the “inactive” burner 200 of the pair injects one or morejets 21 of deflecting gas into the combustion zone 1 with a momentumsufficient to partially deflect the flame 10 from the “active” burner100 back into the combustion zone 1, so that the flame 10 generated bythe “active” burner 100 of the pair does not impinge the burner block201 of the “inactive” burner 200.

In accordance with the invention, the momentum of the deflecting jets 21must however be small enough so as to enable local overheating of theburner block 201 of the “inactive” burner 200 by means of the flame 10generated by the “active” burner 100. This embodiment therefore stillallows efficient heating of the narrow chamber (total fuel combustion)without excessive fouling or thermal damage to the burner block 201 ofthe “inactive” burner 200 and surrounding wall 250.

The deflecting gas can, for example, be an oxidant. In that case, theamount of oxidant injected by the “active” burner 100 can be reducedaccordingly, while still enabling total fuel combustion.

The deflecting gas can also be recycled flue gas. When hot flue gas isinjected as deflecting gas through the burner block 201 of the“inactive” burner 100, the hot flue gas can contribute to the heating upof said burner block 201, in addition to the local overheating caused bythe flame 10 of the “active burner” 100.

As mentioned above, the advantages of oxy-fuel burners over air-fuelburners are generally recognized in the art.

Oxy-fuel flames are generally shorter than air-fuel flames, due to thehigher oxygen-concentration in the oxidant and the lower oxidant volumesinjected. Oxy-fuel flames are therefore in theory particularly suitedfor heating narrow furnaces. In practice however, due to the generallyhigher temperature of oxy-fuel flames and higher radiant heat transfer,particular care has to be taken to prevent thermal damage to the walls250 and to the burner block 201 of the “inactive” burners 200.

Consequently, the use of the embodiment of the present invention wherebythe “inactive” burner 200 of the pair of burners injects one or morejets of gas 21 to deflect the flame 10 back into the combustion zone 1is particularly useful in the case of oxy-fuel burners.

The timing of change-over from step 1 to step 2 and from step 2 to step1 is determined in function of the required flame length and the thermalresistance of the burner block. This timing, i.e. the duration of step 1and of step 2, can be predetermined or can be controlled by measurementsconducted during the process.

-   -   In order to achieve the required limited flame length, the        temperature of the burner block of the “active” burner must be        sufficiently high to achieve the required level of preheating of        the particulate fuel. Change-over must therefore only take place        when the burner block of the inactive burner is sufficiently        overheated. On the other hand, change-over must take place when        the temperature of the “active” burner block has become        insufficient to achieve the required level of reactant        preheating.    -   In order to prevent thermal damage of the burners, change-over        must to take place before the “inactive” burner block reaches a        critical temperature, at which thermal damage of the block or of        metallic parts may occur.

According to a preferred embodiment, the duration of each step ispredetermined. However, the burner block of at least one pair ofburners, and preferably of each pair of burners is preferably equippedwith a temperature sensor, and the method is controlled so thatchange-over takes place when the temperature of an “inactive” burnerblock reaches a critical level, even when the predetermined duration ofthe on-going step has not yet been reached.

The present invention is particularly suited for cross-fired furnacescomprising multiple pairs of particulate fuel burners.

According to one embodiment of the method of the present invention, thefirst burners 100 of the pairs are all positioned on one side of thecombustion chamber and the second burners 200 of the pairs are allpositioned on the opposite side of the combustion chamber so that at anyone time, the “active” burners are all on one side of the combustionchamber and all “inactive” burners are on the opposite side of thecombustion chamber.

According to an alternative embodiment of the method of the inventionillustrated in FIG. 1, some first burners 100 are positioned on one sideof the combustion chamber and other first burners 100 are positioned onthe opposite side of the combustion chamber and vice versa for thesecond burners 200.

According to a preferred embodiment shown in FIG. 1, both said sides ofthe combustion chamber have an alternating succession of first 100 andsecond burners 200 extending in the lengthwise direction of thecorresponding chamber walls 150, 250. In that case, both said sides ofthe combustion chamber present an alternating succession of “active” and“inactive” burners.

EXAMPLE

A calcination furnace of the tunnel type for the production of clinkeris equipped with a dozen pairs of particulate coal oxy-burners. Eachlateral wall of the furnace (taken in the direction of travel of theproduct to be calcined) presents an alternating row of first and secondburners. The first burner of each pair is positioned directly oppositethe second burner of said pair. The burner blocks are made of AZS. Themetal reactant injectors are made of the heat resistant alloy Inconel®600 and are positioned in the rear three quarters of the burner blocks.

Tests were conducted with particulate liquid fuel known as heavy fuel FO2 and with particulate bituminous coal.

1) One-Step Operation of First Burners Only

In a first test, only the first burner of each pair was operated at itsnominal power to generate a flame in the combustion zone situatedbetween the lateral walls. The flames so generated clearly impacted theburner blocks of the second burners causing rapid overheating thereof.The test was terminated before the burner blocks of the second burnersand the surrounding chamber walls suffered thermal damage.

2) Two-Step Operation According to a First Embodiment of the Invention

In a second test, the furnace was operated according to the embodimentof the invention whereby the “inactive” burners do not inject adeflecting gas. Change-over between steps took place when the burnerblock of the “inactive” burners reached a predetermined safe upper limitat which the “inactive” burner block does not suffer thermal damage.

At nominal burner power, the flames generated by the “active” burnerswere found to impinge on the burner blocks of the “inactive” burners.Significant deposition of partial combustion products (soot) on the“inactive” burners was observed and non-negligible overheating of thechamber wall in the vicinity of the blocks of the inactive burners wasdetected. No such problems were observed when the burners were operatedbelow their nominal power.

3) Two-Step Operation in Accordance with a Second Embodiment of theInvention

In a third test, the furnace was operated according to the embodiment ofthe invention whereby the “inactive” burners inject hot recycled fluegas into the combustion chamber so as to deflect the tip of the flamegenerated by the corresponding “active” burner away from the burnerblock and back into the combustion zone.

The burners were operated at nominal burner power.

Change-over between steps took place when the burner block of the“inactive” burners reached the predetermined safe upper limit at whichthe “inactive” burner block does not suffer thermal damage.

Apparently complete fuel combustion was achieved. No substantialdeposition of partial combustion products on the “inactive” burners wasobserved and overheating of the burner blocks of the “inactive” burnersremained localized and did not cause a substantial temperature increaseof the chamber wall in the vicinity of said blocks.

1-19. (canceled)
 20. A method of operating a furnace, comprising thesteps of: a) providing a furnace comprising: a combustion chamber havingwalls defining a combustion zone within the combustion chamber, and atleast one pair of particulate fuel burners, wherein: each pairconsisting of a first particulate fuel burner and a second particulatefuel burner is equipped to inject fuel and oxidant into the combustionchamber; the first and second particulate fuel burners of each paircomprising a burner block having at least one passage for transportingfuel and/or oxidant therethrough towards the combustion chamber; theburner blocks of the first and second particulate fuel burners of eachpair being mounted in the walls of the combustion chamber so as to bepositioned opposite one another across the combustion zone; the firstand second particulate burner of each pair each being adapted togenerate a flame in the combustion zone by: transporting fuel and/oroxidant through the at least one passage of the burner block, andinjecting the oxidant in gaseous form and the fuel in particulate forminto the combustion zone for combustion therein; and b) performingalternating first and second step, wherein during said first step, aflame is generated with the first particulate fuel burner of each pairin the combustion zone that is directed at the second burner block so asto cause local overheating of the second burner block of said pair,while the second particulate fuel burner of each pair does not generatea flame in the combustion zone; and during said second step, a flame isgenerated with the second particulate fuel burner of each pair in thecombustion zone that is directed at the second burner block so as tocause local overheating of the first burner block of said pair, whilethe first particulate fuel burner of each pair does not generate a flamein the combustion zone.
 21. The method of claim 20, wherein: in thefirst step, the second particulate fuel burner of each pair injects adeflecting gas into the combustion chamber so as to partially deflectthe flame generated by the first particulate fuel burner of said pairback into the combustion zone, and in the second step, the firstparticulate fuel burner of each pair injects a deflecting gas into thecombustion chamber so as to partially deflect the flame generated by thesecond particulate fuel burner of said pair back towards the combustionzone.
 22. The method of claim 21, wherein the deflecting gas is flue gasrecycled from the combustion zone.
 23. The method of claim 20, whereinthe fuel injected by the first and second particulate fuel burners ofthe pair is a particulate solid fuel.
 24. The method of claim 23,wherein the particulate solid fuel is selected from the group consistingof particulate coal, pet coke and particulate combustible waste.
 25. Themethod of claim 20, wherein the particulate fuel injected by the firstand second particulate fuel burners of the pair is a liquid fuel. 26.The method of claim 20, wherein the first and second particulate fuelburners of each pair are oxy-fuel burners with oxidant containing atleast 50% by volume of oxygen.
 27. The method of claim 26, wherein theoxidant contains at least 80% by volume of oxygen.
 28. The method ofclaim 26, wherein the oxidant contains at least 90% by volume of oxygen.29. The method of claim 20, wherein: the furnace comprises a multitudeof said pairs of particulate fuel burners; the combustion chamber has afirst wall and a second wall, the first and second walls beingpositioned opposite one another across the combustion zone; and theburner block of the first particulate fuel burner of each pair ismounted in the first wall and the burner block of the second particulateburner of each pair is mounted in the second wall.
 30. The method ofclaim 20, wherein: the furnace comprises a multitude of said pairs ofparticulate fuel burners; the combustion chamber has a first wall and asecond wall, the first and second walls being positioned opposite oneanother across the combustion zone; and a first portion of the multiplepairs has the burner block of the first particulate fuel burner mountedin the first wall and the second particulate fuel burner (200) mountedin the second wall, and the remaining portion of the multitude of pairshas the burner block of the second particulate fuel burner mounted inthe first wall and the first particulate fuel burner mounted in thesecond wall.
 31. The method of claim 30, wherein the first wall has afirst row of burner blocks mounted therein and wherein the second wallhas a second row of burner blocks mounted therein and whereby the firstand second rows are alternating rows of burner blocks of firstparticulate fuel burners and burner blocks of second particulate fuelburners.
 32. The method of claim 30, wherein the first and second arelateral walls of the combustion chamber.
 33. The method of claim 31,wherein the first and second walls are lateral walls of the combustionchamber.
 34. The method of claim 20, wherein the first and second stepshave a predetermined duration.
 35. The method of claim 20, wherein theburner blocks of the first and second burner of at least one pair areequipped with a temperature detector.
 36. The method of claim 35,wherein: the first step is terminated and the second step is commencedon the basis of one or more criteria selected from the group consistingof: the temperature detector of the burner block of the second burnerdetects a temperature exceeding a first predetermined upper limit, thetemperature detector of the burner block of the first burner detects atemperature lower than a first predetermined lower limit, and the firststep has reached a predetermined duration; and the second step isterminated and the first step is commenced on the basis of one or morecriteria selected from the group consisting of: the temperature detectorof the burner block of the first burner detects a temperature exceedinga second predetermined upper limit, the temperature detector of theburner block of the second burner detects a temperature lower than asecond predetermined lower limit, and the second step has reached thepredetermined duration.
 37. The method of claim 20, wherein the furnaceis a tunnel furnace.
 38. The method of claim 20, wherein the furnace isselected from the group consisting of glass melting furnaces, glassfeeders or forehearths, tunnel calcination furnaces and reheat furnaces.